The instant invention relates to the base control of power switches such as, for example, power transistors, Darlington-transistors or cascode-transistors.
The instant invention relates more particularly to a bridge base control circuit such as disclosed in the French patent application 2,542,948, This type of circuit allows a current to flow through the base of the power transistor when one wishes it to be conductive, then, at the switching off of the power transistor, to extract a base current (negative base current in case of a NPN transistor) until the charges stored in this transistor are extracted.
An advantage of the bridge circuits is that it is possible to obtain this result with a single polarity voltage source.
FIG. 1 shows an examplary conventional circuit of a bridge base control. A switch power transistor Tp comprises a base terminal B, an emitter terminal E, and a collector terminal C. A switching bridge comprises four switches T1 to T4 connected between a supply terminal Vcc and a reference terminal M. More particularly, switch T1 is connected between voltage Vcc and the base terminal B, switch T2 is connected between its base terminal B and the reference terminal M, switch T3 is connected between the supply terminal Vcc and the emitter terminal E, and switch T4 is connected between terminal E and the reference terminal M.
Thus, in a first state, switches T2 and T3 are OFF and switches T1 and T4 are ON and a base curent supplied by a current source IB1 flows from the supply voltage Vcc toward terminal B, terminal E and reference terminal M through the ON switches T1 and T4 according to the current path shown by the arrows drawn in solid lines.
At the switching off, switches T1 and T4 are OFF and switches T3 and T2 are ON. Thus, a reverse current flows from supply terminal Vcc through switch T3, terminal E, terminal B, switch T2, toward reference terminal M.
As stated for example, on page 99, in the book published by Societe THOMSON CSF in 1983, entitled "Power transistor in energy conversion", a drawback of this type of base current extracting circuit to which, at the switching off, a reverse voltage is applied between the emitter and the base is that, if voltage Vcc is too high, once the charges stored near the emitter-base junction have been extracted, the emitter-base junction may break down in internal avalanche. As a result, a substantial current is dissipated in the control circuit. Various methods have already been proposed for reducing this drawback, but none can eliminate this drawback in a satisfactory or simple way.